Materials processing using chemically driven spherically symmetric implosions

ABSTRACT

A system for obtaining spherically symmetrical implosion of sample materials by directing radiant ignition energy onto a target which includes a spherically symmetrical core of selected sample material concentrically surrounded by a shell of high explosive material. The resulting implosive compression produces hydrodynamically controlled physical and/or chemical and/or metallurgical transformations of state in the sample material.

REFERENCE TO RELATED APPLICATIONS

This is a continuation of co-pending application Ser. No. 742,035 filedon June 6, 1985, which was a division of application Ser. No. 538,210filed Oct. 3, 1983, now U.S. Pat. No. 4,552,742.

The present invention is directed to materials processing, and moreparticularly to methods and apparatus for the spherically symmetricimplosive compression of matter.

BACKGROUND OF THE INVENTION

It has heretofore been proposed to induce polymorphic transition ofgraphite to diamond by subjecting a starting material to high pressureshock compression. The U.S. Pat. Nos. 3,499,732, 3,653,792 and3,659,972, to Garrett for example, propose the use of shapedelectrically detonated explosive charges to obtain sphericallysymmetrical implosion shock waves for forming diamonds from graphite orfor sintering powdered metals. One significant problem associated withthis technique is one of physical size: the mass of explosive materialinvolved would be on the order of thirty kilograms. An explosion of thissize requires extraordinary containment precautions, and also raisesproblems in connection with recovery and contamination of the sample.Smaller explosive masses cannot be uniformly ignited using theelectrical detonation techniques proposed in the art.

Another significant problem with the above proposed techniques is thelack of control of the implosive pressure afforded by shock compressiontechniques. Yet another problem with the electrically ignited explosivesystem is that of achieving a high degree of spherical symmetry in thedetonating explosive.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention, therefore, to provide a systemand method for implosive compression of materials which reduces the needfor elaborate containment structures, which obtains uniform implosion ofthe sample material, which provides for more ready product recovery,which reduces the introduction of impurities in the sample, and/or whichaffords hydrodynamic control of the implosive pressures in the samplematerial.

Another and more specific object of the invention is to provide a systemand method for obtaining uniform implosion energies of reduced scale,and thereby alleviating the aforementioned deficiencies in the art.

A further object of the invention is to provide a system and method ofthe described type which obtains increased pressure at low temperaturefrom spherical shock compression, and thereby reduces retransformationof the product due to post-shock heating.

Another object of the invention is to provide a system and means foraccurately tailoring the pressure-time history of the implosivelycompressed sample to chemically and/or physically and/or metallurgicallychange the sample's final state.

Briefly stated, the foregoing and other objects of the invention areobtained by uniformly illuminating a spherical target, including asample material surrounded by explosive, with a pulsed laser, ion,electron or microwave beam to ignite the explosive and thereby obtainuniform spherical implosive compression of the material sample. In thepreferred embodiments of the invention, the target comprises a thinignition layer and a shell of high explosive concentrically surroundinga sample material, either with or without a surrounding transparenttamper shell. The target illumination system comprises lenses and/orreflectors for focusing ignition energy uniformly over the targetsurface. Uniform spherically symmetrical high explosive ignition isexpected to be most easily attained using pulsed laser energy and anoptical system designed to provide spherical illumination (hereafterdenoted by "spherical optical system"). Hence, the combination of thepulsed energy and a spherical optical system is used in the preferredembodiment in the following. However, by using many beams, energeticelectron, ion and microwave energy sources could be used in sphericalignition systems. Energetic, pulsed, incoherent light sources incombination with a spherical ignition system may also be used.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objects, features and advantagesthereof, will be best understood from the following description, theappended claims and the accompanying drawings in which:

FIG. 1 is an elevational bisectional view of an implosive compressiontarget in accordance with a basic embodiment of the invention;

FIG. 2 is a schematic diagram of an optical system for focusing laserignition energy onto the target of FIG. 1; and

FIGS. 3-11 are fragmentary sectional drawings illustrating varioustarget constructions in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a target 10 of the present invention as comprising aninner shell 12 of sample material to be implosively compressedcontiguously surrounded by a shell 14 of high explosive material. Theinterior 16 of shell 12 is preferably a vacuum or low pressure air. Eachof the shells 12,14 is of radially uniform thickness, with the overalloutside diameter of target 10 being in the range of several millimetersto several tens of centimeters. The dimensions shown in FIG. 1 areexemplary. The implosive compression of the spherical shell 12 isproduced by the uniform laser ignition of the outer surface of the highexplosive shell 14, whereupon the outer region explodes and theconcomitant reaction force implodes and therefore compresses the"payload". (Here and in the following, "payload" refers to all ofmaterials which are initially driven inward by the explosive forces).The uniform laser light ignition of spherical target 10 may beaccomplished using a number of different methods. One method uses anumber of individual laser beams and focusing lens, a second uses twolaser beams and a spherical system of ellipsoidal mirrors (FIG. 2).

FIG. 2 illustrates an illumination system 20 for directing laserignition energy uniformly over the surface of target 10. System 20includes a pair of centrally apertured concave ellipsoidal reflectors22,24 disposed on a common axis 26 such that the near focus of eachreflector is disposed on axis 26 at or near the midpoint between thereflectors and the far focus of each reflector is disposed on axis 26 atthe central opening of the opposing reflector. A target 10 is suspendedby a thin wire 28 on axis 26 at the common reflector focus. A pair oflenses 30,32 are disposed externally of reflectors 22,24 on axis 26 forfocusing associated collimated beams of laser energy 34,36 through theadjacent reflector aperture to the far focus of the opposing reflector,and thence onto the opposing reflector surface and over target 10. Theincoming laser beams 34,36 may be generated by a suitable pulsed laseramplifier system 60 which directs energy onto a 50/50beamsplitter/reflector 62. The split beams are directed along paths ofequal length to lenses 30,32 by the mirrors 64.

The system 20 thus directs pulsed laser energy substantially uniformlyover the target surface. The basic system 20, to the extent thus fardescribed, is similar to that disclosed in Thomas, "Laser FusionIllumination System," Applied Optics, 14, 6 (June 1975) pp. 1267-1273.The U.S. Pat. Nos. 4,017,163, to Glass, Sigler 4,084,887 and 4,136,926,and Thomas et al 4,161,351, and Brueckner et al, "EllipsoidalIllumination System Optimization for Laser Fusion Experiments," AppliedOptics, 14, 6 (June 1975) pp. 1274-1278 discloses improvements which maybe incorporated into the basic Thomas system.

In operation, opposing collimated beams of pulsed laser energy aredirected with spherical uniformity around the target surface and ignitehigh explosive shell 14. A spherically symmetrical detonation islaunched in shell 14, which both implodes and compresses the material ofshell 12. The implosive detonation accelerates the matter of shell 12toward the center of symmetry, impacting shell 12 upon itself, andthereby raising the density, pressure and temperature in such a way asto induce physical and/or chemical and/or metallurgical changes ofstate. An important advantage of spherical implosion techniques of thetype described lies in the strong increase in density and pressureobtained from spherical convergence at relatively low temperatures, ashas been determined by Guderley, Luffahrtforschung 19, 302, (1942). Thislower temperature, obtained using spherical shock compression inaccordance with the present invention, helps avoid retransformation ofthe product material back to its initial form. Furthermore, the highexplosive material will expand radially outwardly during the compressionof the payload. Thus, no high-density material remains in thermalcontact with the compressed material sample. This helps preventcontamination and aids in cooling. A cup 38 is positioned beneathreflectors 22,24 for collecting the compressed sample. Cup 38 mayinclude an oil or water quench further to reduce retransformation.

Target 10 is constructed using standard machining and/or moldingtechniques. High explosives layers can be cast and/or machined to size.For the formation of diamond from an implosion-induced polymorphictransition in graphite, shell 12 may comprise graphite in pure form orwith catalytic materials as in Cowan U.S. Pat. No. 3,401,019. Ultra-hardboron nitride for industrial machining may be formed byimplosion-induced polymorphic transition of cubic and/or wurtzite formsof boron nitride. The process of the invention may also be employed forimplosion-induced polymerization, sintering of ceramic or refractorymetals, compaction of metal alloys, and possibly for creation ofmetallic hydrogen.

Explosive shell 14 may be constructed of any of a variety of explosives,for example, those known as HMX, PETN, TATB, etc. (See B. M. Dobratz,Lawrence Livermore Laboratory, Report #UCRL-52997). Such explosivestypically possess an energy density of about 5 kJ/g and a mass densityof 1 to 2 g/cc. For a shell 14 having an ID of 6 mm and an OD of 1 cm,explosive energy released would be about 5 kJ. Such an explosion may beeasily contained, and yet will support a "payload" having an ID of 2 mmand a mass of about 1 g. The peak pressure at the target surface can beas high or higher than 200 kiloatmosphere while the "stagnationpressure" at a distance of 13 cm, i.e. at the reflector surfaces, wouldbe only about 10 atmospheres. The pressure impulse would have a decaytime of several microseconds. In order to protect the reflectorsurfaces, sacrificial liners 40,42 made, for example, of softtransparent plastic may be employed.

FIG. 3 illustrates a basic target 10a in accordance with the inventionas comprising a core 13 consisting of a solid sphere of sample material12. Core 13 is contiguously and entirely surrounded by a shell 14 ofexplosive material. The outer surface of core 13 and the thickness ofshell 14 are of respectively uniform radial dimension. In the target 10bof FIG. 4, the core 13 includes a shell of sample material 12surrounding a mandrel 17. Mandrel 17 is a sphere of solid material, suchas aluminum, which prevents implosion pressures in sample material 12from becoming too large and helps control the pressure wave profile intime. FIG. 5 illustrates the target 10 of FIG. 1 wherein the samplematerial shell 12 of core 13 surrounds a void 16.

In the target 10c of FIG. 6, a malleable wave shaping layer or "pusher"shell 48 of iron, for example, is positioned between explosive shell 14and core 13. The function of pusher shell 48 is to obtain desiredtailoring of the hydrodynamic pressure and temperature profiles duringimplosive compression of sample core 13 by extending the time that peakpressure is applied to the sample. In the target 10d of FIG. 7, pusher48 consists of multiple contiguous concentric shells 50 of materialshaving differing density for obtaining more complex pressure andtemperature profile tailoring. This design allows a more nearlyisentropic sample compression, as has been suggested for planar geometryby Lyzenga and Ahrens (in Shock Waves in Condensed Matter, AmericanInstitute of Physics Conference Proceedings #78, 1981). Isentropiccompression reduces the sample temperature rise induced by shock waves.Core 13 in FIGS. 6 and 7 may, of course, be any of those illustrated inFIGS. 3-5.

In order to ignite explosive shell 14, laser energy at a fluence densityof up to 100 joule/cm² from a 25 ns Nd-glass or ruby laser may beemployed. A given explosive material in shell 14 will require a certainamount of ignition energy per unit area, which is readily determined byexperimentation. FIG. 8 illustrates a modified target 10e for loweringthe required ignition energy by providing a thin (on the order of 1000Å) coating or ignition layer 44 of aluminum, for example, over theexplosive shell 14 or a thin layer of chemical explosive having lowignition sensitivity. Use of a thin ignition layer is described inconnection with planar geometries by Yang et al, Applied PhysicsLetters, 19, 473 (1971) and in U.S. Pat. No. 3,812,783. Layer 44 may beformed by standard vacuum deposition or chemical evaporation techniques.For this target design, ignition energy of up to 10 joules/cm² and laserpulse length of 10 ns may be required. FIG. 9 illustrates anothermodified design 10f wherein the ignition layer 44 is disposed betweenexplosive shell 14 and core 13, and explosive shell 14 is transparentfor admitting laser energy. The design of FIG. 9 has the advantage thatthe unburned mass of explosive shell 14 operates to "tamp" the explosiveenergy. Again, any of the cores 13 in FIGS. 3-5 may be employed in FIGS.8-9. Likewise, pusher shells 48 (FIGS. 6-7) may be positioned betweenexplosive shell 14 and core 13 in FIG. 8, or between layer 44 and core13 in FIG. 9.

FIGS. 10 and 11 illustrate "tamped" target designs 10g and 10h. In bothFIGS. 10 and 11, the outer shell or layer (44 in FIG. 11, 14 in FIG. 10)is surrounded by a spherically continuous transparent glass or plastictamper shell 46. Most preferably, tamper shell 46 is sufficiently thickand resilient to absorb the explosive energy without rupture, whichgreatly facilitates both product collection and protection of theillumination optics. SYLGARD 184 marketed by Dow-Corning may beappropriate for construction of shell 46. Such a shell is formed bystandard molding techniques.

It is an important feature of this invention to provide for hydrodynamiccontrol of the pressure and temperature time-histories through the useof layered target structures as shown at 10 through 10h. This procedureis similar in character to the hydrodynamic control used to designinertial fusion targets (see Lindl, U.S. Pat. No. 4,272,320 andBrueckner, U.S. Pat. No. 4,297,165), although the end result is quitedifferent. In the present invention, the sample material is implosivelyprocessed and then recovered, whereas inertial fusion targets aretotally destroyed. In the present invention, such hydrodynamic controlis employed to obtain desired physical, and/or chemical and/ormetallurgical changes of state in the recovered sample material.

The invention claimed is:
 1. A target for exposure to radiant energy toobtain implosive shock compression of materials comprising a sphericalcore including a spherically symmetrical sample of a said materialconcentrically surrounded by a symmetrical shell of high explosivematerial, and multiple spherically symmetrical radially contiguousshells of malleable materials of differing densities between said coreand said shell of explosive material for limiting and extendingapplication of peak pressure to the sample material.
 2. A target forexposure to radiant energy to obtain implosive shock compression ofmaterials comprising a spherical core including a sphericallysymmetrical sample of a said material concentrically surrounded by asymmetrical shell of high explosive material, and a shell of ignitionmaterial abutting with said shell of explosive material and responsiveto said radiant energy for igniting said explosive material.
 3. Thetarget set forth in claim 2 wherein said shell of ignition materialsurrounds said shell of explosive material.
 4. The target set forth inclaim 2 wherein said shell of ignition material is disposed between saidshell of high explosive material and said core, said shell of highexplosive material being transparent to said radiant energy.
 5. A targetfor exposure to radiant energy to obtain implosive shock compression ofmaterials comprising a spherical core including a sphericallysymmetrical sample of a said material concentrically surrounded by asymmetrical shell of high explosive material, and a sphericallysymmetric tamper shell transparent to said radiant energy contiguouslysurrounding said target said core and all shells of said target being insequential radial abuttment.
 6. The target set forth in claim 5 whereinsaid tamper shell is of sufficient thickness and resilience to containdetonation of said shell of explosive material without rupture.
 7. Atarget for exposure to radiant energy to obtain explosive shockcompression of materials, said target consisting of multiple sphericallysymmetrical radially abutting target layers including a central corehaving a spherically symmetrical sample of said material and a shell ofhigh explosive material concentrically surrounding said core.
 8. Thetarget set forth in claim 7 wherein said shell of high explosivematerial includes a spherically symmetrical distribution of ignitionmaterial contiguous with said high explosive material and responsive toradiant energy for igniting said explosive material.
 9. The target setforth in claim 8 wherein said core includes a solid sphere of saidsample material.
 10. The target set forth in claim 8 further comprisingat least one spherically symmetrical shell of malleable material betweensaid core and said shell of explosive material for limiting andextending application of peak pressure to the sample material.
 11. Thetarget set forth in claim 8 wherein said spherically symmetricaldistribution of ignition material comprises a shell of ignition materialcontiguous with said shell of explosive material and responsive to saidradiant energy for igniting said explosive material.
 12. The target setforth in claim 8 further comprising a spherically symmetric tamper shelltransparent to said radiant energy contiguously surrounding said target.13. The target set forth in claim 8 wherein said ignition materialcomprises means responsive to absorption of laser energy for ignitingsaid explosive material.
 14. A target for exposure to radiant energy toobtain explosive shock compression of materials, said target consistingof multiple spherically symmetrical radially abutting target layersincluding a central core having a spherically symmetrical sample of saidmaterial, a shell of high explosive material concentrically surroundingsaid core, said shell of high explosive material including a sphericallysymmetrical distribution of ignition material contiguous with said highexplosive material and responsive to radiant energy for igniting saidexplosive material, and multiple radially contiguous shells of malleablematerial having differing material densities positioned between saidcore and said shell of explosive material for limiting and extendingapplication of peak pressure to the sample material.
 15. A target forexposure to radiant energy to obtain explosive shock compression ofmaterials, said target consisting of multiple spherically symmetricalradially abutting target layers including a central core having aspherically symmetrical sample of said material in the form of a hollowspherical shell of said material and a shell of high explosive materialconcentrically surrounding said core, said shell of high explosivematerial including a spherically symmetrical distribution of ignitionmaterial contiguous with said high explosive material and responsive toradiant energy for igniting said explosive material.
 16. The target setforth in claim 15 wherein the interior of said sample material shell isevacuated.
 17. The target set forth in claim 15 wherein said corefurther comprises a mandrel of a solid sphere of material disposedwithin and contiguously surrounded by said sample material shell.